Welcome
This is the website of the Optical Device Research Group at the National University of Singapore managed by Aaron Danner, an Associate Professor in the Electrical Engineering Department. This site was last updated April 11, 2016.

NUS Solar Helicopter Team Updates

NEWS UPDATE (2016)We have reached 96 cells in the solar quadcopter. Efficiency scales with size; we are almost there!

NEWS UPDATE (2015)The NUS Solar Helicopter Team, an undergraduate student project within the ECE department mentored by Prof. Danner, announces record-setting solar copter efficiency. With 48 solar cells, 130 cm X 130 cm areal size, and a 1.53 kg total aircraft weight, the prototype pictured achieved 61% efficiency in flight (61% solar, 39% battery). Although fully solar-powered (winged) planes were demonstrated years ago by various groups around the world, helicopters require significantly more power to lift their own weight, and a fully flyable solar powered helicopter has not yet been demonstrated that can hover out of the ground effect.

NEWS UPDATE (2014)Prof. Aaron Danner supervised 3 undergraduate teams from NUS participating in the Tech-Factor Challenge sponsored by ST Eletronics. The students worked tirelessly on their projects from August 2013 to May 2014 when they presented their final solar robot prototypes in front of judges. All 3 teams came home with awards.

Overview of Research Projects
Our group is interested in the future of optics and photonics, especially materials and structures that can enhance light-matter interaction, in order to target applications in optical communications, arbitrary optical wavefront generation and detection, quantum information processing, and holography. We are interested in fabrication with nonlinear optical materials and devices, and our group is actively engaged in industrially-relevant research in vertical cavity lasers, laser development for light/heat delivery in magnetic data storage applications, solar cells, and nonlinear optical materials useful for long distance optical fiber communications and on-chip quantum optics with lithium niobate-on-insulator. Materials such as lithium niobate are traditionally rather difficult to work with in terms of practical fabrication challenges, but overcoming some of these challenges would permit photonics-lab-on-chip applications, quantum-optics-on-chip applications, and allow miniaturization of traditionally large external optical modulators. This will be necessary not only for future optical datacom applications, but also for quantum optics applications where programmatic control of single photons will ultimately be required. Our group also hosts the undergraduate student NUS Solar Powered Helicopter Team.

Nonlinear-Optics-On-A-Chip, and Advanced Optical Modulators with Lithium Niobate and other Nonlinear Materials
Lithium niobate is a type of optical material where the refractive index is variable with the applied electric field. This non-linear optical interaction is unfortunately not available in silicon or other commonly used semiconductor materials, but is highly useful for many optics applications, such as creating high speed optical modulators. Lithium niobate devices form core functionality of transoceanic fibre optical networks, enable very high speed electrical-optical signal conversion, and can be used in sensitive detectors to permit construction of sensors impossible with other materials, as they are the only lossless path to material-photon mediation. They are also used in optical frequency doubling applications, and to construct single photon emitters (through parametric downconversion). Commercial lithium niobate modulators currently cost about EUR €5,000 per device, a large sum for a discrete device in an era of large scale integration; such devices have resisted integration and miniaturization due to the large chip area needed to reduce waveguide bending losses in traditional fabrication methods. In fact, this high cost is indicative of practically all optical devices making use of the first order optical nonlinearity.

Our group studies two key challenges associated with the cost of such integration – fabrication of low loss structures such as waveguides in lithium niobate and associated device structures, and reducing the presently prohibitive cost of crystal ion sliced wafers in this material system. We have developed a process where lithium niobate can be etched with an undercut in a monolithic process to create vertical index confinement, a difficult feat in a material that is so difficult even to dry etch, and we also work with lithium-niobate-on-insulater substrates developed in collaboration with an industrial partner. We are currently in the process of applying this method to create nonlinear optical devices on-chip; our group currently (2017) has the lowest published propagation loss (< 0.5 dB/cm) ever reported for etched waveguides. Many of our fabrication methods can be used in other systems, such as barium titanate, which we are also exploring.

INTEGRATED OPTICS ON LITHIUM NIOBATE (2014-2016)These images show various on-chip devices that our group has created in lithium niobate. From left to right: ridge waveguides, a ring resonator (lithium-niobate-on-insulator), a racetrack resonator (lithium-niobate-on-insulator), and a (monolithically fabricated) suspended lithium niobate slab of optically important dimensions.

Interesting Theoretical Questions in Pure Optics
It is an easy problem to start with a lens, and then predict how light will move through the lens, even if the lens is complicated (with a gradient index of refraction, for example). However, the inverse problem is much more difficult - to start by saying "I want a lens that can do X" and then to design the lens that generates the requested behavior. For example, let's take a single basic request and see how complicated the situation can become: "Create a lens where all light ray paths are closed." This is difficult! The well-known Principles of Optics textbook by Max Born and Emil Wolf mentions only two such lens examples. (These are now known as "absolute optical instruments" because they region entire regions of space stigmatically.) In 2017 along with our Czech colleagues at Masaryk University, we determined how to design such lenses a priori through separation of the Hamilton-Jacobi equation; there are in fact infinitely many such lenses.
These are very interesting theoretically, because there are some interesting deep relationships between ray optics and wave optics that assert themselves in these lenses. A related problem in classical mechanics is analagous, with some important caveats: "What are all the potentials that can result in closed particle orbits?" The famous Bertrand Theorem says that there are only two, the harmonic oscillator, and the Newtonian potential. This explains the stability of orbits (in astronomy), the stability of atomic orbits (the Coulomb interaction), and these potentials happen to be spherically symmetric. But are there other stable potentials outside the Bertrand Theorem? Yes, there are infinitely many, if spherical symmetry is not required, and is also related to separability of the Hamilton-Jacobi equation although in a much more restrictive way than in the optical case. One such example is shown in the image. While our group is primarily interested in unusual theoretical optics problems, we enjoy exploring interesting physics problems in general. We are now pursuing some interesting theoretical topics in imaging.

INTERSTING THEORETICAL OPTICS PROBLEMSIn the left image is a gradient-index lens where all light rays follow curved, closed paths. This is an example of an absolute optical instrument. The frequency spectrum is also completely degenerate, which is a related property that our group has proven. In the right image, we introduce the concept of "force tracing", to show how radiation pressure is distributed throughout a graded index object. Traditionally, radiation pressure distribution has been very time-consuming and troublesome to calculate because of the difficulty in solving the Maxwell stress tensor. The "force tracing" technique is orders of magnitude faster computationally, and is analagous, in some sense, to the use of geometrical optics to approximate wave optics.

FUN OPTICS SIMULATIONSHave you ever wondered what a swimming pool full of negative index water would look like? Here we have side-by-side simulations! In the negative (n = -1.33) index case, the bottom surface of the pool would appear to be floating above the water's surface.

Solar Cell Packaging Glass: To Texture or Not to Texture? (That is the question...)
We aim to enhance the collection efficiency of solar cells through surface texturing while simultaneously making the surface anti-dust. This is possible because texturing can simulateously alter the surface energy, making a surface either hydrophobic or hydrophilic, and can also alter a surface optically. Our group developed a non-lithographic method to create pseudo-random nanostructures on the glass surface. It was observed that the nano-patterning improved transmission of planar glass for a wide 120-degree range of incidence angles. Improvement of 1% in (absolute) power conversion efficiency was also observed in solar modules packaged with the glass. After 3 months of outdoor testing, the difference was even greater due to the anti-dust property (2% reduction in efficiency for standard modules versus only 0.3% for modules with the nano-patterned glass). Thus, antireflective and self-cleaning glass can show significant improvement in efficiency of fixed-mount solar module installations, and may play a greater role in future photovoltaic applications.

SOLAR CELL GLASSGlass texturing can improve solar cell cleanliness and efficiency. Modules were tested on top of the Engineering (EA) building at NUS.

Vertical Cavity Surface-Emitting Lasers (VCSELs)
Our group is fabricating and characterizing high power densely-packed arrays of VCSELs for applications which require a very fast rise time and extremely high brightness. We have also recently explored the feasibility of employing VCSELs as the light source in next-generation Heat Assisted Magnetic Recording (HAMR).

NANO-SCALE APERTURES FOR HEAT-ASSISTED MAGNETIC RECORDINGBecause the aperture needed for light delivery is much smaller than the working wavelength (~50 nm versus 850 nm), a circular aperture is no longer the optimal shape. Our group has studied various aperture shapes and optimized the design, along with VCSELs that could potentially be used to replace edge emitters in such a light delivery system. The left image shows one of our lasing VCSELs. The middle image shows an array of test aperatures. The right picture shows our groups pump-probe setup, to test writing of magnetic bits while under laser illumination.

2:39 PM 11/8/2017
Recommended Reading and Links for Students
Photonic crystals, and other periodic structures can be modeled in frequency space using the planewave expansion method. Prof. Danner's
tutorial on the plane wave expansion method
may prove useful to students interested in learning how photonic crystal band diagrams can be calculated from first principles. While commercial and free tools are available to calculate bands of photonic crystals, a similar method can be used to model electronic bands in semiconductors. A step-by-step
description of the pseudopotential method
may be useful for students trying to learn the method. The link also includes a downloadable Mathematica file that implements the method. For students interesting in getting started, the following reading material in sequence is recommended (with a basic knowledge of electromagnetic fields assumed).

Recommended technical books and articles (in order from novice to advanced):